A ship made of pykrete is probably a lot easier to scuttle than one made out of concrete. You just let it melt.

That can take forever in the arctic, as mentioned in the article...

Another one from WWI was the SS Atlantus. Although scrapped after 2 years service it was used successfully to transport troops back from the Euro theatre after the war. I remember seeing its 3-piece remains sitting in the waters off Cape May, NJ during a visit there as a little kid.

With concrete canoes, wonder if they ever experimented with Roman seawater concrete, which generates heat when it reacts with water. Heated hulls supposedly exhibit less friction which may give a slight edge in a race.

Regular Portland cement goes off under water too - it is exothermic as it reacts but it doesn't continue to generate heat. The "heated hull" would only be during the initial setting when the cement was weak and prone to cracking.

A heated hull riding on a bed of steam sounds like an interesting [and very high energy cost] engineering challenge though!

Cf. https://en.wikipedia.org/wiki/Cement

Mind game: posit construction of one of these cement canoes with Roman seawater cement (but with topology incorporating hollow cylinders in whatever is designated the stern). Data mining tidbit: 'For underwater structures, lime and volcanic ash were mixed to form mortar, and this mortar and volcanic tuff were packed into wooden forms. The seawater instantly triggered a hot chemical reaction. The lime was hydrated—incorporating water molecules into its structure—and reacted with the ash to cement the whole mixture together.” * Back to the canoe race. Once the flotation test is passed, incorporate sealed mortar canisters into cylinders and when race is underway expose mortar to sea water. Result: jet thrust for a time with residue morphing undetected (maybe) into the original primary construction material. Odds are you'd be disqualified but it might be a fun feasibility study.

*http://www.bloomberg.com/bw/articles/2013-06-14/ancient-roma... ... reaction reportedly reaches temperature approaching 900C.

I like the way you're thinking but I'm doubting you can generate any useful thrust from this.

> The Selma developed a crack, nobody knew how to repair it

Pykrete, however, appears to be repairable with seawater, a refrigeration unit, and a hose.

The concrete canoe community is doing rather well though. http://concretecanoe.org/